Try a Restart.
Press and hold the Sleep/Wake button for a few seconds until the red "slide to power off" slider appears, and then slide the slider. Press and hold the Sleep/Wake button until the Apple logo appears.
Resetting your settings
You can also try resetting all settings. Settings>General>Reset>Reset All Settings. You will have to enter all of your device settings again.... All of the settings in the settings app will have to be re-entered. You won't lose any data, but it takes time to enter all of the settings again.
Resetting your device
Press and hold the Sleep/Wake button and the Home button together for at least ten seconds, until the Apple logo appears. Apple recommends this only if you are unable to restart it.
Or if this doesn't work and nobody else on the blog doesn't have a better idea you can contact Apple.
Here is a link to their contacts with most of the information below.
I spent another session on the phone with T-Mobile. They told me the issue was a known issue but was resolved already. Except when the representative left me a voicemail the notification still did not come though. Lol. I was told if it was not resolved within 72 hours to try calling them again. They put in another ticket to get the issue resolved and the next evening it started working again. I hope you were able to get yours working again by now. If not, call T-Mobile again. I do to think it was an Apple issue. Thank you Apple engineers for your excellent work! Keep it up!